1. Field of the Present Invention
The present invention relates to motion measurement, and more particularly to a motion inertial measurement unit in micro size that can produce highly accurate, digital angular increments, velocity increments, position, velocity, attitude, and heading measurements of a carrier under dynamic environments.
2. Description of Related Arts
Generally, an inertial measurement unit (IMU) is employed to determine the motion of a carrier. In principle, an inertial measurement unit relies on three orthogonally mounted inertial angular rate producers and three orthogonally mounted acceleration producers to obtain three-axis angular rate and acceleration measurement signals. The three orthogonally mounted inertial angular rate producers and three orthogonally mounted acceleration producers with additional supporting mechanical structure and electronic devices are conventionally called an Inertial Measurement Unit (IMU). The conventional IMUs may be cataloged into Platform IMU and Strapdown IMU.
In the platform IMU, angular rate producers and acceleration producers are installed on a stabilized platform. Attitude measurements can be directly picked off from the platform structure. But attitude rate measurements cannot be directly obtained from the platform. Moreover, there are highly accurate feedback control loops associated with the platform.
Compared with the platform IMU, in the strapdown IMU, angular rate producers and acceleration producers are directly strapped down with the carrier and move with the carrier. The output signals of the strapdown angular rate producers and acceleration producers are expressed in the carrier body frame. The attitude and attitude rate measurements can be obtained by means of a series of computations.
A conventional IMU uses a variety of inertial angular rate producers and acceleration producers. Conventional inertial angular rate producers include iron spinning wheel gyros and optical gyros, such as Floated Integrating Gyros (FIG), Dynamically Tuned Gyros (DTG), Ring Laser Gyros (RLG), Fiber-Optic Gyros (FOG), Electrostatic Gyros (ESG), Josephson Junction Gyros (JJG), Hemisperical Resonating Gyros (HRG), etc. Conventional acceleration producers include Pulsed Integrating Pendulous Accelerometer (PIPA), Pendulous Integrating Gyro Accelerometer (PIGA), etc.
The processing method, mechanical supporting structures, and electronic circuitry of conventional IMUs vary with the type of gyros and accelerometers employed in the IMUs. Because conventional gyros and accelerometers have a large size, high power consumption, and moving mass, complex feedback control loops are required to obtain stable motion measurements. For example, dynamic-tuned gyros and accelerometers need force-rebalance loops to create a moving mass idle position. There are often pulse modulation force-rebalance circuits associated with dynamic-tuned gyros and accelerometer based IMUs. Therefore, conventional IMUs commonly have the following features:
High cost,
Large bulk (volume, mass, large weight),
High power consumption,
Limited lifetime, and
Long turn-on time.
These present deficiencies of conventional IMUs prohibit them from use in the emerging commercial applications, such as phased array antennas for mobile communications, automotive navigation, and handheld equipment.
New horizons are opening up for inertial sensor device technologies. MEMS (MicroElectronicMechanicalSystem) inertial sensors offer tremendous cost, size, and reliability improvements for guidance, navigation, and control systems, compared with conventional inertial sensors.
MEMS, or, as stated more simply, micromachines, are considered as the next logical step in the silicon revolution. It is believed that this coming step will be different, and more important than simply packing more transistors onto silicon. The hallmark of the next thirty years of the silicon revolution will be the incorporation of new types of functionality onto the chip structures, which will enable the chip to, not only think, but to sense, act, and communicate as well.
Prolific MEMS angular rate sensor approaches have been developed to meet the need for inexpensive yet reliable angular rate sensors in fields ranging from automotive to consumer electronics. Single input axis MEMS angular rate sensors are based on either translational resonance, such as tuning forks, or structural mode resonance, such as vibrating rings. Moreover, dual input axis MEMS angular rate sensors may be based on angular resonance of a rotating rigid rotor suspended by torsional springs. Current MEMS angular rate sensors are primarily based on an electronically-driven tuning fork method.
More accurate MEMS accelerometers are the force rebalance type that use closed-loop capacitive sensing and electrostatic forcing. Draper""s micromechnical accelerometer is a typical example, where the accelerometer is a monolithic silicon structure consisting of a torsional pendulum with capacitive readout and electrostatic torquer. Analog Device""s MEMS accelerometer has an integrated polysilicon capacitive structure fabricated with on-chip BiMOS process to include a precision voltage reference, local oscillators, amplifiers, demodulators, force rebalance loop and self-test functions.
Although the MEMS angular rate sensors and MEMS accelerometers are available commercially and have achieved micro chip-size and low power consumption, however, there is not yet available high performance, small size, and low power consumption IMUs.
A main objective of the present invention is to provide a core inertial measurement unit, which can produce digital highly accurate angular increment and velocity increment measurements of a carrier from voltage signals output from the specific angular rate and acceleration producers thereof, so as to obtain highly accurate, position, velocity, attitude, and heading measurements of the carrier under dynamic environments.
Another objective of the present invention is to provide a core inertial measurement unit (IMU) which successfully incorporates the MEMS technology.
Another objective of the present invention is to provide a core inertial measurement unit, wherein output signals of angular rate producer and acceleration producer are exploited, and are preferably output from emerging MEMS (MicroElectronicMechanicalSystem) angular rate sensor arrays and acceleration sensor arrays. These outputs are proportional to rotation and translational motion of the carrier, respectively. Compared with a conventional IMU, the present invention utilizes a feedforward open-loop signal processing scheme to obtain highly accurate motion measurements by means of signal integration, digitizing, temperature control and compensation, sensor error and misalignment calibrations, and dramatically shrinks the size of mechanical and electronic hardware and power consumption, meanwhile, obtains highly accurate motion measurements.
Although the present invention can use existing angular rate devices and acceleration devices, the present invention specifically selects MEMS angular rate devices and acceleration devices to assemble a core IMU, wherein the core IMU has the following unique features:
(1) Attitude Heading Reference System (AHRS) Capable Core Sensor Module.
(2) Miniaturized (Length/Width/Height) and Light Weight.
(3) High Performance and Low Cost.
(4) Low Power Dissipation.
(5) Shock resistant and vibration tolerant.
(6) Dramatic Improvement In Reliability (microelectromechanical systemsxe2x80x94MEMS).
Another objective of the present invention is to provide a core IMU rendering into an integrated micro land navigator that has the following unique features:
(1) Miniature, light weight, low power, and low cost.
(2) AHRS, odometer, integrated GPS chipset and flux valve.
(3) Integration filter for sensor data fusion and zero velocity updating.
(4) Typical applications: automobiles, railway vehicles, miniature land vehicles, robots, unmanned ground vehicles, personal navigators, and military land vehicles.
Another objective of the present invention is for the core IMU to function as aircraft inertial avionics, which has the following unique features:
(1) Rate Gyro
(2) Vertical Gyro
(3) Directional Gyro
(4) AHRS
(5) Inertial Navigation System
(6) Fully-Coupled GPS/MEMS IMU Integrated System
(7) Fully-Coupled GPS/IMU/Radar Altimeter Integrated System
(8) Universal vehicle navigation and control box.
(9) North Finding Module.
Another objective of the present invention is to provide a core IMU to function as a Spaceborne MEMS IMU Attitude Determination System and a Spaceborne Fully-Coupled GPS/MEMS IMU Integrated system for orbit determination, attitude control, payload pointing, and formation flight, that have the following unique features:
(1) Shock resistant and vibration tolerant
(2) High anti-jamming
(3) High dynamic performance
(4) Broad operating range of temperatures
(5) High resolution
(6) Compact, low power and light weight unit
(7) Flexible hardware and software architecture
Another objective of the present invention is to provide a core IMU to form a marine INS with embedded GPS, which has the following unique features:
(1) Micro MEMS IMU AHRS with Embedded GPS
(2) Built-in CDU (Control Display Unit)
(3) Optional DGPS (Differential GPS)
(4) Flexible Hardware and Software System Architecture
(5) Low Cost, Light Weight, High Reliability
Another objective of the present invention is to provide a core IMU to be used in a micro pointing and stabilization mechanism that has the following unique features:
(1) Micro MEMS IMU AHRS utilized for platform stabilization.
(2) MEMS IMU integrated with the electrical and mechanical design of the pointing and stabilization mechanism.
(3) Vehicle motion, vibration, and other interference rejected by a stabilized platform.
(4) Variable pointing angle for tracker implementations.
(5) Typical applications include miniature antenna pointing and tracking control, laser beam pointing for optical communications, telescopic pointing for imaging, airborne laser pointing control for targeting, vehicle control and guidance.
Another objective of the present invention is to provide a core IMU, wherein a support bracket and shock mount are incorporated to install the core IMU on the carrier to mitigate vibration and shock on the core IMU to obtain improved performance.
Another objective of the present invention is to provide a core IMU, wherein a LCD (liquid crystal display) module with a backlight button and a reset button is incorporated to function as an inertial palm navigator, which is designed to provide a user with inertial position, velocity, and attitude information in a wide variety of commercial applications, as following: automobile tracking systems, automobile positioning systems, personnel watercraft, snowmobiles, motorcycles, ships, antenna pointing and stabilization systems, general aviation, survey, etc.
Another objective of the present invention is to provide a core IMU, wherein for standard commercial operating conditions, a user of the core IMU for hand held operation without need of a GPS device, can obtain his attitude and position from any location. The user initiates operation of the device after power up by inputting a bench mark or initial position through the quick touch of the reset button. This action establishes the starting position and zero velocity point. The core IMU enables a user to perform the following applications:
(i) Free Inertial: Free Inertial mode is the only mode of operation available when a radar update is not available. During free inertial operation, a user can expect exponential error growth with time.
(ii) Doppler Radar Aiding: During radar aiding operation, a user can expect a small position drift proportional to the distance traveled.
(iii) While the user is in a vehicle: During ground vehicle motion, a user can expect a position drift proportional to the distance traveled. During use in a vessel, a user can expect a drift proportional to the distance traveled combined with the vector sum of the unknown velocity of the current.
During all modes of operation, stable attitude information is available in the form of a Pitch, Roll and Heading output of data in the form of a set of numbers on the display. Attitude is available after the initial alignment period of three minutes. The core IMU has a limit of a maximum input angle rate. If the user exceeds this limit for any amount of time, attitude output accuracy may be lost for a few seconds. Attitude accuracy for heading depends upon the calibration of the Earth""s magnetic field detector heading sensor. If the heading sensor has been calibrated for the local magnetic field, full performance can be obtained. If the local magnetic field is not calibrated, errors as large as ten degrees can be found.